The invention relates to low noise amplifiers, and more particularly to circuitry and techniques for increasing the linearity and reducing distortion in low noise amplifiers without excessively increasing circuit complexity and/or power dissipation therein.
By way of background, resistance in series with the source electrodes of a pair of differentially coupled MOSFETs is the main source of noise in a differential amplifier. The prior art circuit shown in FIG. 1 illustrates a very low noise differential amplifier. The reason that its noise is low is that there is no resistor connected between the source electrodes of MOS field effect transistors (MOSFETs) Q1 and Q2. The gain of this amplifier is given by the expression G=g.sub.m R.sub.L, where g.sub.m is the transconductance of the pair of differentially connected input MOSFETs Q1 and Q2, and is very non-linear. Unfortunately, since this low noise circuit is very non-linear it introduces a great deal of distortion as it amplifies the differential input signal to produce a differential output signal.
FIG. 2 shows a prior art solution to the distortion of the circuit in FIG. 1. That solution is to connect a gain-setting resistor 15 of resistance R.sub.S between the source electrodes of the input MOSFETs Q1 and Q2. Constant current source MOSFETs Q3 and Q4 replace the single constant current source MOSFET Q3 in the circuit of FIG. 1. The gain of the circuit of FIG. 2 is given by the expression G=g.sub.m R.sub.L /(1+g.sub.m R.sub.S), which simplifies to G.apprxeq.R.sub.L /R.sub.S if R.sub.S is made large. Unfortunately, the solution of FIG. 2 to the distortion problem of the circuit of FIG. 1 greatly increases the noise. (Those skilled in the art know that a random noise voltage e is produced in a resistor of resistance R; e is equal to the square root of 4kTBR, where k is Boltzmann's constant, T is the temperature in degrees Kelvin, and B is the bandwidth of the circuit including the resistor.
FIG. 3 shows a prior art solution to both the distortion problem of the circuit of FIG. 1 and the noise problem of the circuit of FIG. 2. The solution is to add two feedback operational amplifiers 18 and 19 as shown. The gain of the circuit of FIG. 3 is given by the expression G.apprxeq.A.multidot.R.sub.L /(A+1).multidot.R.sub.S, which simplifies to G.apprxeq.R.sub.L /R.sub.S for a large value of the gain A of the operational amplifiers 18 and 19. This in effect allows the resistance R.sub.S of the gain setting resistor 15 to be divided by roughly the gain A of the operational amplifiers 18 and 19. Therefore R.sub.S can be small enough that both low noise operation and low distortion operation are achieved. However, this is achieved at the expense of (1) additional conventional operational amplifier circuitry that is required to implement the two operational amplifiers 18 and 19, and (2) substantial additional power dissipation in the two operational amplifiers.
In the past, the input impedance of differential input, differential output amplifiers has been reduced by their feedback circuitry. This has made it necessary to introduce additional high impedance input buffers between the input signals and the input terminals of the basic differential amplifier stages.
It would be desirable to provide a low power, low distortion differential amplifier that dissipates less power than has been achieved by the prior art. It also would be desirable to provide such a low power, low noise, low distortion differential amplifier having very high input impedance and differential outputs without use of feedback circuitry of a kind that reduces the input impedance.